1. Field of the Invention
This invention relates to polymeric compositions which are particularly suited for use as electrical insulation coatings on copper wires in underground cables. In these underground installations, the insulated wires are surrounded by a packing of petrolatum or other soft hydrocarbon medium which is used to keep out water but has been found to accelerate oxidation and heat degradation of the polymeric coatings. The present invention is directed toward the development of a stabilizer system which provides superior insulation coatings for use in this complicated environment. More particularly, it relates to such insulating compositions in which the polymeric component is derived largely from propylene and three different stabilizers are used therein in certain critical and unconventional concentrations and proportions thereby providing surprisingly long lasting protection of the insulation against degradation.
2. Description of the Prior Art
It is well known that .alpha.-olefinic polymers such as polyethylene, polypropylene and the like, including various copolymers and/or blends of same, are susceptible to degradation due to heat, oxidative attack and other environmental influences. Literally hundreds of additives have been proposed by those skilled in the art as having the capability to reduce the severity of such degradation in hydrocarbon based materials, and many have been used with some success to increase the stability of polymeric products under normal environmental conditions. However, soon after isotactic polypropylenes and other crystalline polymers derived predominantly from propylene became commercially available and were used to insulate electrical conductors made of copper and other metals, it was discovered that the use of the conventional heat and antioxidant stabilizer additives in such insulating compositions in the accepted amounts and combinations failed to provide adequate service life.
After these failures had been traced to catalytic interactions caused by the presence of copper or other metals in the electrical conductors, considerable progress was made in alleviating the premature failures of these insulating materials by the inclusion in the propylene based polymers used therein of various metal deactivator compounds such as chelating agents. For example, the use of certain organic hydrazide or hydrazine compounds in such a manner is disclosed in U.S. Pat. Nos. 3,438,935 and 3,484,285. It appears from these two prior art disclosures that the best results are obtained by using said compounds in a concentration of about 0.5% based upon the weight of the polymeric material, i.e. in amounts about equal to the concentration of the primary antioxidant stabilizing additive contained in said polymeric material.
Unfortunately, even when such improved polypropylene-based compositions were employed in waterproof underground cable assemblies, the level and duration of stability of the insulation coatings became insufficient, apparently because of the extractive action of the petrolatum or other soft hydrocarbon filler material used as the waterproof packing around the insulated electrical conductors. For example, based upon accelerated thermal oxidation laboratory tests in Differential Scanning Calorimeters at 200.degree.C in the presence of copper, crystalline polymers derived largely from propylene and containing 1% by weight of a hindered phenolic type antioxidant and 1% by weight of a hydrazide compound show an induction period of only about 10 minutes after extraction for 8 hours at 85.degree.C in U.S.P. petrolatum compared to about 100 minutes before extraction.